1. Field of the Invention
The invention relates to a display device employing a liquid crystal material between a first substrate provided with row or selection electrodes and a second substrate provided with column or data electrodes, in which overlapping parts of the row and column electrodes define pixels. The display device further employs drivers for driving the column electrodes in conformity with an image to be displayed, and for driving the row electrodes which, in the operating condition, sequentially supply groups of p row electrodes with p mutually orthogonal signals. Such display devices are used in, for example, portable apparatuses such as laptop computers, notebook computers and telephones.
2. Description of the Related Art
Passive-matrix displays of this type are generally known and, for realizing a high number of lines, they are increasingly based on the STN (Super-Twisted Nematic) effect. An article by T.J. Scheffer and B. Clifton “Active Addressing Method for High-Contrast Video Rate STN Displays”, SID Digest 92, pp. 228-231 describes how the phenomenon of “frame response” which occurs with rapidly switching liquid crystal materials is avoided by making use of “Active Addressing”. In this method, all rows are driven throughout the frame period with mutually orthogonal signals, for example, Walsh functions. The result is that each pixel is continuously excited by pulses (in an STN LCD of 240 rows: 256 times per frame period) instead of once per frame period. In “multiple row addressing”, a (sub-)group of p rows is driven with mutually orthogonal signals. Since a set of orthogonal signals, such as Walsh functions, consists of a plurality of functions which is a power of 2, i.e. 2S, p is preferably chosen to be equal thereto as much as possible, i.e. generally p=2s (or also p=2s−l). The orthogonal row signals Fi(t) are preferably square-wave shaped and consist of voltages +F and −F, while the row voltage is equal to zero outside the selection period. The elementary voltage pulses from which the orthogonal signals are built up are regularly distributed across the frame period. In this way, the pixels are then excited 2s (or (2s−1)) times per frame period with regular intermissions instead of once per frame period. Even for low values of p such as p=3 (or 4) or p=7 (or 8) the frame response appears to be suppressed just as satisfactorily as when driving all rows simultaneously, such as in “Active Addressing”, but it requires much less electronic hardware.
However, it appears that, notably for Walsh functions, the frequency content of the functions from a complete set of functions is greatly different. Since the dielectric constant of liquid crystalline material is frequency-dependent, this may cause the liquid crystalline material to react differently at different positions in, for example, a matrix display, dependent on the image contents. This leads to artefacts in the image such as different forms of crosstalk.